NCCAM

The National Center of Complementary and Alternative Medicine (NCCAM) at the National Institutes of Health, classifies Reiki as a biofield therapy — the medical use of subtle energy fields in and around the body for positive health effects.

See the NCCAM website about Reiki: http://nccam.nih.gov/health/reiki/

From the NCCAM Website:

Uses (of Reiki)
According to the 2007 National Health Interview Survey, which included a comprehensive survey of CAM (Complementary and Alternative Medicine) use by Americans, more than 1.2 million adults had used an energy healing therapy, such as Reiki, in the previous year. The survey also found that approximately 161,000 children had used an energy healing therapy in the previous year.
People use Reiki for relaxation, stress reduction, and symptom relief, in efforts to improve overall health and well-being. Reiki has been used by people with anxiety, chronic pain, HIV/AIDS, and other health conditions, as well as by people recovering from surgery or experiencing side effects from cancer treatments. Reiki has also been given to people who are dying (and to their families and caregivers) to help impart a sense of peace.

Effects and Safety
Clients may experience a deep state of relaxation during a Reiki session. They might also feel warm, tingly, sleepy, or refreshed.
Reiki appears to be generally safe, and no serious side effects have been reported.

Reiki Research

NCCAM
American Holistic Nurses Association
Reiki in New Hampshire Hospitals
What is Reiki?
History of Reiki
Additional Research